According to the last nationally representative survey of American families, over 16% of couples experienced an incident of physical assault in the previous year. The mental health ramifications of intimate violence is great, as victims can suffer from post traumatic stress disorder, depression, and alcoholism. To date, the most widely accepted theory to explain violence in intimate relationships is social learning theory, which posits that since violence against intimates tends to run in families, children must learn how to be violent through watching their parents and through being reinforced for their own aggressive behaviors. However, a strict social learning account of intimate violence takes into account only environmental influences on this behavior. Behavioral genetic research has shown that many behaviors also have genetic influences, and without a behavioral genetic study of intimate violence, we cannot be sure if its causes are solely environmental. The goal of the proposed twin study is to investigate if the familial resemblance in intimate violence is do to genetic influences, shared environmental influences, or both. Maximum likelihood model fitting analyses will be conducted on the twins' aggressive behaviors in intimate relationships to estimate the relative contributions of genes, shared environments, and non-shared environments to individual differences in these behaviors. If genetic influences are found, multivariate model-fitting techniques will be used to investigate whether genetic influences on these aggressive behaviors overlap with genetic influences on personality. Two hundred adult twin pairs will complete questionnaires regarding their use of aggressive behaviors in intimate relationships, personality, and childhood exposure to violence.